This section is intended to introduce various aspects of the art, which may be associated with examples discussed in the Detailed Description section. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of those examples discussed in the Detailed Description section. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Reservoir Quality (RQ), as applied to oil and gas reservoirs, is a measure of the porosity and permeability. For clastic reservoirs, mainly consisting of quartz grains, a significant loss of RQ can occur as the rock matures under the conditions of deep burial and elevated temperature. The loss of RQ is identified with the growth of quartz overgrowths under deep burial and elevated temperature.
The occurrence of micron-sized quartz coatings on the original quartz grains can inhibit further quartz growth and prevent the reduction of RQ. These thin coatings are referred to as microquartz grain coatings. Microquartz grain coatings can be identified through examination of rock cores with an optical or electron microscope. Regions with microquartz grain coatings are high RQ zones and geologic mapping can be used to estimate their extent. “High,” when used to characterize RQ zones, is a relative characterization. The context of various factors, discussed further below, provide the needed context.
When high RQ is encountered and there is reason to expect that quartz cements should have blocked the pore space, for example deep burial, an examination of the rocks is undertaken. First, using an optical microscope, the clastic grains are examined for grain coats, with microquartz coats a frequent type. Abundant microquartz grain coats are associated with high RQ. If the optical examination fails to identify such coats, the samples are examined at higher magnification using electron microscopy. However, to extend this finding throughout the reservoir requires examination of many hundreds of samples from cores. Both vertical and horizontal RQ variation is needed. Finally, construction of a map that defines the boundaries of the high RQ zones is also needed.
In some deep, hot reservoirs, the burial history can be used along with a quartz growth model to predict the expected amount of quartz cementation, and therefore RQ loss. Cementation is the hardening and welding of clastic sediments (e.g., quartz) (those formed from preexisting rock fragments) by the precipitation of mineral matter in the pore spaces. If sample logs indicate higher RQ, then it is assumed that some mechanism has preserved this porosity. However, models that can relate these observations to reservoir distributions require subsequent optical and electron microscopy examination of the samples identical to that for the previous approach. This creates the need for drilling wells to supply the necessary rock samples.